Humanity has always been curious about their surroundings since the first cave dweller discovered how to start a fire. The earliest nomads were constantly challenging themselves to see what was behind the next mountain, or across the next sea. Throughout their history, mankind as a whole has been focused on discovery of the unknown, even if the definition of unknown was merely about understanding the nature of insect life spans. However, even before discovering the New World, great philosophers and scientists were looking to the stars to answer the bigger questions like, "Why were we here?" and "Where did we come from?" While religions would try and answer those questions with vague answers about a divine power, other people looked to more tangible explanations that only science could provide.
Even though it took several thousand years, Humans finally reached for the stars to start getting answers. The moon landing was indeed just a first step for a growing interest of the cosmos to the people on Earth. Telescopes became a commonplace household item, and schools educated the importance of discovery and technological advancement. However, for a time the thirst for knowledge to know what was up there stalled, while people were more concerned with material wealth. A new era of space flight and colonial expansion began when the societies on Earth realized that one planet alone could no longer sustain their needs, and in 2069 the first Lunar colony was established. Things moved at a rapid pace after that crowning achievement and others quickly followed suit, grabbing other planets in the Sol system to start living on. These new bases allowed Humans to explore new environments with a fresh sense of wonder and excitement that had been lost after centuries of living on Earth.
This enthusiasm probably led to the first discovery of ancient alien life on Mars. On January 4th, 2076, two miners digging for mineral deposits located a large structure that they knew could not be Human. It was made with a shiny metal that they had never seen before, and the angles making up the structure seemed impossible to modern Human engineering. The archaeological find shook Humanity to it's foundation, suddenly people were aware that they were truly not alone in the universe. Scientists from all over the Sol system gathered on the red planet, like it was a holy site, hoping that this mysterious structure would finally give them answers to their most burning questions. Unfortunately, the site only provided more questions that needed to be addressed as soon as possible. The language was easy enough for experts to decipher in it's written form but, they couldn't immediately connect garbled voice recordings to the text which was becoming a problem.
Back on Earth, radical religious communities were starting to rise up and openly protest looking into the alien technology, seeing it as the devil's work. They believed that knowing the truth about these mysterious beings would inevitably destroy their faith and way of life forever. What if the god or gods they had worshipped since the beginning of time were actually just aliens that had mastered space flight before them? It would invalidate the power of the gods and remove the fear some people had to defy religious practices. Tensions only grew worse overtime to the point that many people started fearing an inter-planetary civil war that had the potential to wipe Humanity from the face of the Milky Way. Terrorist attacks became common once again, and secret societies rose in popularity, some even went to the extremes of trying to take down entire governments across the globe to try and stop the progress on Mars.
The growing conflicts back home didn't shake scientists' resolve to learn more, and soon they came up with a name for the beings they were researching, the Protheans. Three years after the Prothean data cache had been found on Mars, scientists realized that most of the data was actually referring to a network of faster than light travel devices they called 'Mass Relays.' According to the data, these devices were capable of transporting ships across entire star systems within hours rather than decades and, as if that miracle wasn't enough, one just happened to be in the Sol system. This stunned people on Earth and their natural ambition to expand started to take hold of the public conversation. Governments started making new powerful coalitions to try and be the ones to control travel through their Mass Relay, even though it's exact location hadn't been determined yet.
Radical religious movements took full advantage of the growing chaos and on December 3rd, 2079, they successfully assassinated the President of the United States at the time. The group boldly tried to throw the guilt of the assassination onto a geopolitical rival of the U.S. but, their involvement was discovered before a global war could begin. However, a smaller war with the terrorists started as a result of the assassination, ending in the cult's ultimate destruction. In the aftermath of the chaos, Humanity decided that if they were going to travel the stars they needed a unified vision to present to the galaxy and thus created the Systems Alliance of Humanity. Some criticized the formation of the new government as a way for the rich and powerful to create a, "One World Order," but the majority would see the need for the Alliance soon enough.
During the time of the Alliance's rise, scientists were able to triangulate data for the Mass Relay in Sol, realizing that the planet of Pluto was actually just a giant ice formation hiding the Relay. It wasn't until 2130 that Pluto was able to be broken apart because of it's distance from Earth, but once the last layer of ice had been removed from the massive Prothean structure, Humans were already volunteering to be the first to travel through it. The honor to lead the journey ultimately landed on Admiral John Grissom, who fought in the earlier civil war against religious radicals in America. He personally led the first wave of around 300,000 Humans out of the Sol system, and into the wider Milky Way galaxy. At first people had mixed feelings about the trip since every world they colonized appeared to have no indication that the Prothean's, or any other alien civilization still existed. This caused many to believe that they had been too late and were indeed the last living beings in the universe. It wasn't as if extinctions were unheard of on Earth, and so the general consensus was that some galactic event caused the Protheans to be wiped out making Humanity the last true superpower in the galaxy.
This assumption would cost Humanity greatly when they made first contact with intelligent alien life on the desert planet, Shanxi. As modern day historians tell the story, the colony on Shanxi created a visual beacon for itself when the colonists were celebrating, Alliance Day, on April 16th of 2142 by setting off fireworks. They did not realize that the planet was already inhabited before their arrival. The species living on the planet later identified as the Turians, a militaristic species, saw the fireworks as poorly aimed projectiles towards their own settlements and responded with deadly force against the colony. Admiral Grissom was not prepared for the sudden, organized, and methodical attack and died saving as many people as he could from the colony. The death was a huge blow to the Alliance's colonial command structure, and the damage was compounded by the fact that communications back to Earth hadn't been fully implemented yet. Unfortunately, the tragedy on Shanxi led to Admiral Jack Connig Williams taking control of the military for all of the colonies. Unlike Grissom, Williams had no sense for widespread military command and in a desperate act of short sightedness used all of their nuclear weapons on Shanxi thinking that the planet was the only place where Turians lived, he was wrong.
The Turians were an extremely military minded race and saw Williams' actions as an act of war that they later called the Relay 314 Incident. The Turian military machine quickly mobilized preparing to exterminate Humanity, while also trying to avoid the watchful eyes of the rest of the more peaceful intergalactic community. After several bloody battles on Human controlled colonies, it became clear that the Turians would eventually win with pure brute force and a numbers advantage. However, communications with Earth became a reality and a second wave of Alliance reinforcements arrived on June 8th, 2142 tipping the balance of power to a stalemate. The Turians couldn't hide their actions forever, and eventually their military movements caught the attention of the Citadel Council who became aware of the conflict and demanded an immediate cease fire between the two races. Both sides were punished for the conflict but, in the next forty years Humanity has been able to become a galactic superpower.